During operation, the heat load is very intense inside a rocket combustion chamber. The walls of the combustion chamber must be cooled efficiently to prevent melting, or in other ways damaging or destroying the structure. The most common way to cool the chamber wall is by convection cooling. Accordingly, cool fuel, and even oxidizer is used in the cooling process.
The service life of such chambers is often a problem. Much care must be taken to ensure proper function. Inspection and repair during development and use of the engines is costly. The service life very much depends on the temperature level of the wall structure closest to the flame. The temperature gradient over the cooling channels generates thermal stress. The elevated temperatures degrade the material properties. Therefore, the service life is strongly influenced by the temperature. Reduction of the temperature by 100° F. leads to about three times increase in service life and 10 times increase in creep life.
The intense heat load leads to stratification of the coolant. The coolant closest to the hot wall is heated which results in a temperature increase. The viscosity of the coolant is lowered leading to increased flow speed closest to the heated wall. Thus, the coolant is stratified with sharp temperature gradients. A large portion of the coolant is only heated to a low temperature level, reducing the efficiency of the cooling system. The temperature difference in the coolant may be in the order of 600-700° F. At the outer side of the cooling channel, near the outlet end, the coolant may still have the inlet temperature of 60° F.
It has been proposed to enlarge the cooling surface of the cooling wall, for example by having longitudinal fins along the inside the channels, however, the fins need to have some height to penetrate the thermal boundary layer. The coolant flow speed will be slowed down in the gap between the fins in case they are made high and close together. Therefore, the increase in heat transfer is limited with this measure. Also, the bottom of each fin needs to be sharp to give room for a large number of fins. The sharp bottom is perpendicular to the first principle stress. The channel bottom represents an important stress concentration. The fins are delicate to manufacture. The width of the channels at the throat area is in the order of 1.0 mm, which means that the maximum width of one of three fins is 0.3 mm and the tip of the fin becomes infinitely thin.
Also, it has been proposed to make heat transfer more effective by increasing the channel wall surface roughness to generate turbulence in the coolant flow. The surface roughness increases the vortexes at the wall, but the effect is small with a very low viscosity fluent as hydrogen.
JP 60048127 teaches the use of a twisted steel band inside a horizontal cooling channel to force a secondary flow to avoid stratification. This method is proposed for application in nuclear plants at horizontal pipes in reactors, intermediate pumps, heat exchangers and inlet nozzles of steam generators. The steel band may lead to hot spots at the hot side and overheating of the material due to a reduced flow of coolant in the channel.